1. Field of the Invention
This invention relates to a piezoelectric device used as a driving source of an injector.
2. Description of the Related Art
An injector (fuel injection device) of an internal combustion engine, or the like, of an automobile is constituted in such a fashion that an opening/closing state of a fuel passage is changed over when a three-way valve or two-way valve connected to a common rail storing a high-pressure fuel is operated, to thereby change a pressure condition applied to a nozzle needle, and the fuel is injected when the nozzle needle is opened.
A solenoid valve is generally used as a driving source for operating the valve body. In contrast, attempts have been made to use a stacked-type piezoelectric device as the driving source so as to precisely control the fuel injection state by controlling more precisely the driving source.
However, an injector using the piezoelectric device for the driving source has not yet been put into practical application.
Japanese Unexamined Patent Publication (Kokai) No. 7-321383, for example, discloses a conventional piezoelectric device that has a coating film of an insulating resin on outer peripheral side surfaces of a stacked-type piezoelectric device. This film is directed to improve moisture resistance and to protect the piezoelectric device from an impact.
On the other hand, a piezoelectric device for an injector must have extremely high-speed response to open and close a valve at a high speed. In the piezoelectric device for an injector, therefore, charging and discharging is repeated within a very short time, and a large current flows. For this reason, if the coating film of the insulating resin is merely formed on the outer peripheral side surfaces of the piezoelectric device as in the prior art device, the coating film prevents the radiation of the heat generated by the energy consumed inside the piezoelectric device. As heat stays inside the piezoelectric device and the temperature rises, a drop in performance of the piezoelectric device may occur and, in the worst case, the piezoelectric device may be broken.
When the rise of the internal temperature of the piezoelectric device is large, the necessary energy becomes large, too, and this imparts a burden on a control circuit with the result that the size of the control circuit must be increased.
In view of the problems of the prior art described above, it is therefore an object of the present invention to provide a piezoelectric device for an injector having less internal exothermy and a smaller consumed energy.
According to one aspect of the present invention, there is provided a stacked-type piezoelectric device, for an injector, to be built into the injector, including a metal case outside the side surfaces of the piezoelectric device, wherein an insulating member having an electric insulating property is interposed between the side surfaces of the piezoelectric device and an inner surface of the case, one surface of this insulating member is bonded to either the side surfaces of the piezoelectric device or the inner surface of the case, and at least a part of the other surface of the insulating member is in contact with the side surfaces or the inner surface.
In the piezoelectric device according to the present invention, the insulating member is bonded to the side surfaces of the piezoelectric device or is in contact with the side surfaces. Therefore, heat occurring inside the piezoelectric device is directly transferred to the insulating member without passing through an air layer.
The insulating member is in contact with the metal case or is bonded to the metal case. Therefore, heat transferred to the insulating member is directly transferred to the case without passing through an air layer.
Here, since the case is made of a metal, efficient heat radiation, by utilizing high thermal conductivity, can be accomplished. Therefore, the heat transfer from the piezoelectric device to the insulating member is promoted and a temperature rise of the piezoelectric device can be suppressed.
In the present invention, in particular, either the boundary surface of the insulating member with the piezoelectric device or the boundary surface of the insulating member with the case is in a bonded state, and the other surface is merely in contact. Therefore, in comparison with the construction where these two boundary surfaces are bonded, deformation of the case due to thermal expansion of the insulating member can be suppressed. In other words, since one of the boundary surfaces is merely in contact, the two members can move while maintaining mutual heat transferability. Therefore, when the insulating member undergoes thermal expansion, its movement at the contact surface can escape stress, that would otherwise invite deformation of the case, to a certain extent, and deformation of the case can thus be suppressed.
As described above, the present invention can provide a piezoelectric device for an injector which consumes less energy due to a restriction on the internal temperature, or the like.
According to another aspect of the present invention, there is provided a stacked-type piezoelectric device to be built in an injector, wherein side surface electrodes are disposed on the side surfaces of the piezoelectric device, respectively, an external electrode for establishing electric conduction with outside is provided to each side surface electrode, and thermal conductivity of the external electrode is at least 15 W/mxc2x7K.
As described above, the piezoelectric device for the injector according to the present invention has an external electrode the thermal conductivity of which is limited to the specific value described above. Therefore, a rise of the internal temperature of the piezoelectric device can be suppressed. On the other hand, when the thermal conductivity of the external electrode is less than 15 W/mxc2x7K, the heat radiation effect of the external electrode cannot be sufficiently obtained.
According to still another aspect of the present invention, there is provided a piezoelectric device for an injector having a built-in piezoelectric device as a driving source, wherein the piezoelectric device has a metal case outside side surface thereof, an insulating member having an electric insulation property is disposed between the surfaces of the piezoelectric device and an inner surface of the case, one surface of the insulating member is bonded to either the side surfaces of the piezoelectric device or the inner surface of the case, and at least a part of the other surface of the insulating member is in contact with the other surface, and wherein the case is so constituted as to directly or indirectly come into contact with a cooling fluid inside the injector.
In the injector according to the present invention, the cooling fluid can directly or indirectly cool the metal case, and the heat from the piezoelectric device can be efficiently radiated. Therefore, a high-speed response of the piezoelectric device can be secured and a control circuit can be rendered compact. Accordingly, the injector of the present invention is compact and has an excellent performance.
According to still another aspect of the present invention, there is provided a method of producing a stacked-type piezoelectric device to be built in an injector, comprising the steps of covering the outside of side surfaces of the piezoelectric device with an insulating member, and bonding the insulating member and the side surfaces of the piezoelectric device; and inserting the piezoelectric device covered with the insulating member into a metal case.
In the production method according to the present invention, the insulating member is disposed in advance on the piezoelectric device, and the piezoelectric device is then disposed inside the case. In this way, bonding adhesion between the insulating member and the piezoelectric device and the contact state between the insulating member and the case can be achieved easily in a relatively narrow space between the piezoelectric device and the case, and the occurrence of defects such as voids can be suppressed when the insulating member is disposed.
In consequence, an excellent piezoelectric device for the injector described above can be produced.
According to still another aspect of the present invention, there is provided a method of producing a stacked-type piezoelectric device to be built in an injector, comprising the steps of covering an inner surface of a metal case with an insulating member, and bonding the insulating member to the inner surface of the case; and inserting the piezoelectric device into the case covered with the insulating member.
This production method can also provide the same function and effect as that of the production method described above. In other words, according to this production method, the insulating member is disposed in advance on the inner surface of the case, and the piezoelectric device is then disposed inside the case. In this way, bonding adhesion between the insulating member and the case and the contact state between the insulating member and the piezoelectric device can be easily achieved inside a relatively narrow space between the piezoelectric device and the case, and the occurrence of defects such as voids can be suppressed when the insulating member is disposed. In consequence, this production method, too, can produce an excellent piezoelectric device for the injector.
The present invention may be more fully understood from the description of preferred embodiments of the invention as set forth below, together with the accompanying drawings.